Field
This disclosure relates to acoustic metamaterial that relates to energy generation using hybrid resonant metastructures.
Background
Acoustic metamaterials are manufactured or synthetic structures that aim to achieve acoustic/elastic properties which are not available in tradition materials. In particular, negativity in effective dynamic mass density was demonstrated in various different designs. Materials with negative acoustic properties present a negative mass density and bulk modulus, and therefore a negative index of refractivity. Negative effective bulk modulus was also realized in fluid channels with cavity resonators. Other effects such as focusing, image magnifying, acoustic cloaking, total absorption were also realized experimentally. Currently, simultaneous negativity in both effective mass density and bulk modulus was only achieved by a composite structure of membranes and pipe with side-holes.
The past decade witnessed the arrival of acoustic metamaterials which expanded the horizon of sound wave manipulations. Phenomena such as extreme attenuations, cloaking, sub-diffraction imaging and manipulations, low frequency total absorption of airborne sound, were conceived and subsequently realized. Many of these breakthroughs benefit from the emergence of an approach which reduces a complex system to a fictitious homogenous material that is characterized by a small set of effective constitutive parameters. It is desired to apply a similar approach to tackle the problem of acoustic absorption of low frequency sound, a traditionally very difficult problem.
The absorption of airborne acoustic waves has long been a problem with both fundamental and practical interest. Various techniques such as porous/fibrous bulk materials, micro-perforations, resonant structures, and random scatterers, have been employed to improve sound absorption performance of either certain particular frequencies, or over a broad frequency band. These approaches seek to damp acoustic energy by increasing the dissipation coefficient, delaying the propagation of the wave, or boosting the energy density within the absorber.
The dissipation of sound is essentially the conversion of kinetic energy of air particles to heat. Ultimately this must be carried out via a combination of viscosity and friction; i.e., dissipative energy is generally proportional to the square of the first time-derivative of displacement (in linear systems) times the viscosity coefficient. Despite this, a large viscosity coefficient may not necessarily lead to large absorption, since it may simultaneously cause impedance mismatch between air and the absorber. In such case a good portion of the incident energy would be reflected at the interface. Therefore, only when the viscosity and the impedance of the whole system fit certain criteria can the absorber reach its optimal performance.
One of the characteristics of metamaterials is that, according to their basic design, they can reach a point of super-absorption, in which the platelet or mass vibrates at maximum amplitude. It would be desired to be able to extract energy absorbed by the metamaterials in order to increase the capacity of the metamaterials to absorb sound or other vibrational energy.